Nutrient retention within the hyporheic zone of a low order sandstone stream (Weidlingbach, Lower Austria)

Abstract

In this study at the Weidlingbach, a fourth order sandstone stream, the hydrological exchange between surface and interstitial water as well as the retention potential of the hyporheic zone for inorganic nutrients were studied. For this purpose, we performed shortterm nutrient additions (ammonium, nitrate, phosphate) at a 75 m long, third order study reach between May and August 2004. Additionally, hydrological and chemical parameters to estimate water exchange (VHG, hydraulic conductivity, conductivity, oxygen concentration) were measured in the stream and interstitial water. Sediments at the study reach could be divided into sites with either upwelling or restricted water exchange (low permeability, positive VHGs, low oxygen content) and sites which were characterized by a significant downwelling of surface water. Hyporheic nutrient uptake was measured at these latter sites. The nutrient with the highest retention efficiency was ammonium, followed by phosphate and nitrate. Vertical uptake curves indicated that the retention efficiency was highest in the sediments at the surface. These results imply that, at the Weidlingbach, the hyporheic zone does not considerably contribute to stream retention and that the surface sediments and the surface water play a greater role in nutrient cycling.